Reduction Kinetics of NO ₃ – vs NO ₂ – toward Ammonia Synthesis on a Magnesium Single-Atom Electrocatalyst

Ammonia is an essential chemical and an emerging carbon-free hydrogen carrier, yet its conventional Haber–Bosch synthesis is energy-intensive and CO 2 -emitting. Electrochemical reduction of nitrogen oxyanions offers a sustainable alternative; however, achieving high selectivity toward ammonia while suppressing the competing hydrogen evolution reaction (HER) has been predominantly realized using transition-metal-based catalysts. In contrast, alkaline-earth metals remain largely unexplored due to their intrinsic instability and the long-standing challenge of stabilizing them as single-atom catalysts. Herein, we demonstrate the catalytic potential of an alkaline-earth single-atom system by developing a chlorophyll-derived magnesium single-atom catalyst (Mg 1 /N–C-650) featuring atomically dispersed Mg–N 4 centers, synthesized via a simple, environmentally benign, and cost-effective route. Electrochemical studies reveal a pronounced electrolyte-dependent kinetic divergence in nitrogen oxyanion reduction. While both nitrate and nitrite are selectively converted to ammonia, nitrite reduction exhibits markedly enhanced intrinsic kinetics, with Mg 1 /N–C-650 exhibiting a 2.74-fold higher turnover frequency compared to nitrate media, while delivering an ammonia production rate of up to 1851.07 ± 62.87 μmol h –1 mg –1 cat. (−0.25 V) under optimal reaction conditions. Density functional theory calculations reveal that the Lewis-acidic Mg–N 4 centers effectively polarize N–O bonds, promote direct nitrite hydrogenation, and intrinsically suppress HER due to unfavorable H adsorption. This work establishes alkaline-earth magnesium single-atom catalysts as a previously underexplored platform for sustainable ammonia electrosynthesis.